Flexible packaging is widely used for food, non-food, and pharmaceutical applications. Flexible packaging uses a wide range of different types of materials including various types of plastic films, paper, and aluminum foil. The plastic films include various types of polyolefins, polyesters, and polyamides. The films may be various combinations of homopolymers, copolymers, and polymer blends. The films may be a single layer or may be coextruded in multiple layers. The films are also commonly coated, metalized, or otherwise treated to enhance the performance of the resulting package. Packaging materials are selected based on a variety of factors including desired barrier properties, appearance, cost, physical feel, printability, sealing properties, easy open features, and reclosing features.
Two main classes of flexible packaging materials are: 1) mono-web packaging, which includes a mono-web of a coextruded film; and 2) laminated packaging. Laminate packaging is often desired due to the fact that it is advantageous to combine two or more webs in order two obtain the desired properties of the resulting package. Reasons for using laminated packaging constructions include: 1) to contain the graphics between layers in order to provide protection and enhanced appearance; 2) to maintain product freshness by taking advantage of the barrier properties of the individual layers; 3) to combine a heat stable web for printing with a heat sealable web for sealing the package; 4) to provide desired feel and handling properties to maximize consumer appeal; 5) to enhance the package strength in order to maintain integrity for filling, shipping, and consumer handling.
Several different technologies are used to bond the layers used in laminated packaging. Two classes of laminating technology are extrusion lamination and adhesive lamination. Extrusion lamination involves melting and depositing a layer of thermal plastic resin such as polyethylene between two webs of packaging materials. The different types of adhesives currently used to laminate flexible packing materials include: 1) one component solvent base; 2) two component solvent base; 3) one component water base; 4) two component water base; and 5) two component solventless.
Solvent base adhesives have inherent limitations that include: 1) emission of volatile organic compounds (VOCs); 2) high cost of solvent incineration or recovery equipment; 3) flammability; and 4) analysis and control of residual solvents in the package.
Water base adhesives have inherent limitations that include: 1) the need for extended drying equipment; 2) the effect of heat used in drying on thermally sensitive packaging films; 3) variable drying rates dependant on ambient humidity levels; and 4) difficulty in starting and stopping due to adhesive drying on the application equipment.
Any two component system (solvent base, water base, or solventless) has inherent disadvantages that include: 1) the need for accurate mixing of the two components; 2) limited pot life of the mixed components; and 3) the time delay (typically 2 to 5 days) required for the two components to react to achieve the final adhesive properties. Other limitations associated with two component solventless adhesives include: 1) the need for heated application equipment; and 2) residual toxic aromatic amines, which are byproducts of isocyanate based curing systems.
Radiation-curable adhesives can potentially offer numerous advantages over these other flexible packaging laminating adhesives. They may offer: 1) stable one-part compositions; 2) little or no VOCs; and 3) full adhesive performance immediately upon cure. UV curable laminating adhesives require at least one layer of packing material that is sufficiently transparent to allow penetration of UV light to cure the adhesive. EB curing has the added advantage of being able to penetrate opaque or printed packaging materials in order to cure the adhesive.
The main challenge in the development of radiation-curable laminating adhesives are: 1) two provide bonding and chemical resistance that is adequate for desired packaging application; and 2) have low odor, taint, and migration to allow packaging of food and pharmaceutical products.
Radiation-curable materials such as inks and coatings are generally based on relatively low molecular weight reactive monomers and oligomers. The components are designed to be converted to high molecular weight polymers upon UV or EB irradiation. High conversions of the low molecular weight components can be achieved; however, some residual amount of monomer or oligomers normally remains. These residual components can be responsible for odor, taint, and migration issues in the packaging. The art of radiation-curable inks and coatings does not address the same problems associated with flexible laminate packaging materials, and, thus, one skilled in the art would not be motivated to look to the art of radiation-curable inks and coatings when addressing the problems associated with radiation-curable adhesives for use in laminates.
A discussion of the issues associated with the use of radiation-curable materials in food packaging applications may be found in PCT Application number WO 02/081576 (Chatterjee), which is incorporated herein by reference. The compositions disclosed by Chatterjee contain water, which is displaced from the ink or coating upon radiation-curing. This cannot be done with a laminating adhesive since the water would be trapped between two layers of packaging materials and, thus, Chatterjee is not helpful in addressing the problems associated with radiation-curable adhesives for use in making laminates.
In radiation-curable laminating adhesives, the residual low molecular weight components are initially found within the cured adhesive, which is located between two layers of packaging materials. Some types of packaging materials, such as aluminum foil, are good barrier materials and are effective for preventing migration of low molecular weight components in to the food or pharmaceutical product. Other packaging materials, such as polyolefin based materials, are known to be less effective barriers to migration of low molecular weight organic compounds. Thus, there is a need for a radiation-curable adhesive material that when suitably cured exhibits substantially reduced migration through layers in a laminated flexible packaging material.
Flexible packaging materials also have problems with delamination of the layers during normal use, especially when the package contains aggressive liquids or certain aggressive food products. Delamination can also be an issue during processing or the package. This can include the addition of closures, filling, sealing, and heat processing. Thus, there is a need for a radiation-curable adhesive material that when suitably cured exhibits sufficient adhesion to prevent delamination of the layers during normal use.